Because the poorest people tend to die from easily preventable diseases, addressing poverty is a relatively cheap way to save lives. War, by contrast, is extremely expensive. This article argues that, since states that wage war could alleviate poverty instead, poverty can render war unjust. Two just war theory conditions prove relevant: proportionality and last resort. Proportionality requires that war does not yield excessive costs in relation to the benefits. Standardly, just war theorists count only the direct costs: the death and destruction wrought by war. This article argues that it can sometimes be appropriate to add the opportunity costs of a failure to alleviate poverty. Last resort is the condition that there must be no better alternative means of achieving the same just end for which the war is waged. This article argues that there are some cases in which alleviating poverty may constitute a better alternative. These are cases in which the most fitting description of the just end for war is sufficiently general that poverty alleviation offers a means to pursue it. The idea that poverty can sometimes render war unjust has, to date, been largely overlooked. It is, nevertheless, an idea with profound implications since, once taken seriously, war becomes much harder to justify. Wars that, in every other respect, seem just may prove disproportionate or unnecessary given the alternative of alleviating poverty.